Planar magnetic assemblies, i.e., transformers and inductors, are used widely in high current/low voltage switching power supplies operating from 40 kHz to 1 MHz. A typical transformer is a major part of the power converter, which either steps voltage up or down depending on the application. In higher power converters either the primary winding or the secondary winding of the transformer has to carry AC current over 100 amperes RMS and sometimes up to 500 amperes RMS or more. Filter inductors, on the other hand, have to carry DC current, but the values can also be quite high. In both cases, the planar magnetic assembly has to be connected to semiconductors, which either switch, or rectify the currents. The impedance of this connection generates power losses, additional electromagnetic interference and can be difficult to reduce.
One type of prior art high power planar transformer has copper standoffs connecting the planar layers. The layers are made of flat copper leadframes, which must be connected in parallel to reduce total DC and AC resistance of the winding. Designers normally select the thickness of the leadframes in the range of 10 to 32 mils for transformers because of the skin effect. The skin effect describes a reduction of electric field density in metal conductors as a function of waveform frequency. For example, a copper conductor carrying a 250 kHz current exhibits approximately a 37% reduction in electric field density from its surface to the depth of 5.2 mils. This depth is different for different metals and characterizes a specific skin depth for a given metal at a given frequency. Because of the skin effect, planar transformers are more efficient at higher operating frequencies than their conventional magnetic wire wound counterparts. However, even flat planar conductors do not solve the problem of sufficient copper cross-sectional area for heavy current windings. In many applications paralleling just two leadframes does not yield low enough winding resistance. Accordingly three or more leadframes must be connected. This connection must also solve a problem of electrical impedance of mechanical interface. While providing a convenient screw-type connection, standoffs have three drawbacks. First, the copper standoff must be mechanically swaged and then soldered to the leadframes. Swaging may put part of the standoff above the surface of the planar leadframe thus making electrical connection between the two flat surfaces questionable. Second, in many cases transformers are custom designed to meet specific requirements. Therefore, the distance between leadframes varies widely from model to model so that it becomes impractical to design and manufacture different height standoffs for every model. Third, connecting three or more leadframes in parallel using standoffs, while possible, presents a difficult manufacturing problem.
In another prior art embodiment, L-shaped copper terminals are soldered to multiple planar leadframes. This configuration provides a more flexible connection because a single length L-shaped terminal can accommodate almost any variances in distances between leadframes. After transformer assembly, the L-shaped terminal is inserted in slots provided for this purpose in the leadframes and soldered in place providing a flat terminal with an aperture ready for a screw-type connection to semiconductors and other components. However, there are two major problems with this approach. First, a single L-shaped terminal has to provide at least the same copper cross-sectional area, as all leadframes it connects in parallel. Increasing the L-shaped terminal's thickness will not solve the problem in an optimum way due to the skin effect. Second, soldering a very thick copper L-shaped terminal to multiple leadframes becomes a difficult manufacturing task due to the heat-sink effect of massive copper on the soldering joint.
An additional problem occurs for both standoff and single L-shaped terminals. When connecting three or more leadframes in parallel, both L-shaped terminals and standoffs bring the screw-type interface point either to the top or bottom of the planar transformer. The AC current in the leadframes tends not to equalize with most of the current flowing in those leadframes which are the closest to the connecting screw. The further away from the connecting screw that a leadframe is located in the planar stack, the less current will flow in it. This phenomena will increase AC resistance and, therefore, reduce efficiency.
Accordingly, there has been a long felt need for a flexible and low impedance terminal system which is capable of delivering large currents to semiconductor switches or rectifiers, easy to install and use, cost-effective, and improves the efficiency of planar magnetics.